The present invention relates generally to radio communication, and more specifically to a system for adjusting the gain of a transmit signal to an optimal level for a desired cumulative distribution level.
A communication system is formed, at a minimum, of a sending station and a receiving station interconnected by way of a communication channel. Information generated at the sending station is communicated to the receiving station over the communication channel. A wide variety of different types of communication systems have been developed and are regularly utilized to effectuate communication of information between sending and receiving stations.
Advancements in communication technologies have permitted the development and construction of new types of communication-systems. A radio communication system is exemplary of a type of communication system which has benefited from advancements in communication technologies. A communication signal formed between the sending and receiving stations of a radio communication system is defined over a radio link. Because a radio link is utilized to form the communication signal, the conventional need of wireline connections between the sending and receiving stations is obviated. Thus, increased mobility is inherent in a radio communication system in contrast to a conventional wireline system.
The communication capacity of a radio communication system, however, is sometimes constricted as a result of bandwidth limitations. Only a limited amount of the electromagnetic spectrum is typically allocated to be used by a particular radio communication system and upon which communication channels can be defined. Communication capacity increase of a radio communication system is, therefore, sometimes limited by such allocation of bandwidth. Increase of the communication capacity of the radio communication system, therefore, is sometimes only possible by increasing the efficiency by which the allocated spectrum is used.
Digital communication techniques are among the advancements in communication technologies which have advantageously been implemented in various communication systems. Digital communication techniques permit the bandwidth efficiency of communications over a communication channel in a communication system to be increased. Due to the particular need to efficiently utilize the bandwidth allocated in a radio communication system, the use of such digital techniques is particularly advantageously utilized in a radio communication system.
An important operation in digital communications is transforming a digital transmit signal into an analog transmit signal. A digital-to-analog converter (DAC) is typically used to perform such a transformation. The DAC obtains the digital signal over a discrete number of bits and outputs an analog representation of the signal. Since the DAC uses a finite number of bits, care must be taken that the input digital signal does not exceed the full-scale limit of the DAC. If the digital input signal increases beyond the DAC""s maximum output value, the analog signal remains at the maximum value, even though the digital signal is greater than the corresponding analog value. Such a situation is often referred to as xe2x80x9cclippingxe2x80x9d because the peaks of the digital signal are flattened to the maximum value of the DAC in the corresponding analog signal.
Clipping can result in signal degradation, poor adjacent channel power ratio (ACPR), and information loss. One way to avoid clipping is to control the amplification level of the digital transmit signal so that the full-scale limit of the DAC is never reached.
Avoiding any amount of signal clipping, however, can often increase quantization errors inherent in DACs; especially if the peaks of the digital signal are infrequent and much greater than the average transmit signal amplitude. This is because less of the DAC""s resolution is devoted to the most prevalent portion of the input digital signal. Quantization errors in digital communication systems can often lead to signal distortion and poor signal-to-noise ratios. Thus, it may be advantageous to strike a careful balance between some amount of clipping and some degree of quantization error in the digital transmit signal of CDMA communication systems.
Accordingly, the present invention provides a method, system and computer program for controlling the gain of a spread spectrum transmit signal such that a desired cumulative distribution level of the signal is associated with the full-scale limit of the DAC.
One aspect of the present invention is a method for controlling the amplification of an input transmit signal at a desired cumulative distribution level in a transmitter. The input transmit signal is coupled to a digital-to-analog converter and includes a plurality of independent spread spectrum channels. The spread spectrum channels have associated individual channel gains. The method includes an approximating operation to approximate the total variance of the input transmit signal. A determining operation determines an amplification gain corresponding to the desired cumulative distribution level using an amplification function dependent on the total variance of the input transmit signal.
Another aspect of the invention is a system for controlling the amplification gain of an input transmit signal in order to operate at a desired cumulative distribution level in a radio transmitter. The input transmit signal includes a plurality of independent spread spectrum channels, with the spread spectrum channels having individual channel gains. The system includes an amplifier receiving the input transmit signal and outputting an amplified transmit signal. The amplifier is configured to amplify the input transmit signal in response to a gain control signal. A digital-to-analog converter is coupled to the amplified transmit signal, and a gain control unit is coupled to the gain control signal. The gain control unit is configured to approximate a total variance of the input signal and to adjust the gain control signal based on an amplification function dependent on the total variance of the input transmit signal.
Yet a further aspect of the invention is a computer program for controlling amplification of an input transmit signal in a transmitter in order to operate at a desired cumulative distribution level. The input transmit signal is coupled to a digital-to-analog converter and includes a plurality of independent spread spectrum channels. The spread spectrum channels have individual channel gains. The computer program is configured to approximate a total variance of the input transmit signal and determine an amplification gain corresponding to the desired cumulative distribution level using an amplification function dependent on the total variance of the input transmit signal.